Project Summary: The endoplasmic reticulum (ER) hosts such critical events as protein folding, lipid synthesis, and maintenance of calcium homeostasis. ER stress occurs when increased demand for these activities is elicited by diverse environmental challenges, metabolic fluctuations, or pathological conditions. When the capacity to meet prolonged or severe demand is exceeded, cell death is often the result. Much of the existing literature has focused on apoptotic mechanisms occurring at mitochondria. Many therapeutic strategies in protein misfolding diseases are directed towards correcting protein misfolding or targeting apoptotic mechanisms. Even as these approaches are important, we have discovered a new pathway in yeast that allows adaptation to ER stress. The main hypothesis in this proposal is that mitochondrial biogenesis is an adaptive response to ER stress by protecting cells against accumulation of reactive oxygen species (ROS) and cell death. Our hypothesis is supported by preliminary results showing that increased mitochondrial mass induced by a low dose of ER stressor results in increased resistance to a subsequent higher dose. In Specific Aim 1 of the proposal we will determine the mechanism by which ER stress induces mitochondrial biogenesis. We have evidence that adaptive response requires the retrograde signaling pathway that communicates mitochondrial needs to the nucleus. We will determine whether mitochondrial-ER contact sites are required for communicating ER stress to mitochondria. In addition, new genetic screens are proposed to identify new components required for mitochondrial response to ER stress. In Specific Aim 2, we will determine whether adaptation to ER stress occurs similarly in mammalian cells. We propose studies in thyroid (PCCL3) and pancreatic (INS-1) cell lines as these secretory cells have exquisite sensitivity to ER stress. Of great biological and medical significance, our aims should lead to discovery of new therapeutic targets for impaired ER stress response in numerous diseases.